Electrical discharges occur in the electrical machines for many reasons. The electrical discharges are harmful and they tend to damage the machine in different ways. Three main causes are partial discharges of the windings, the bearing currents and sparking of the commutator. All these phenomena have been thoroughly investigated and there have been many proposals how to prevent and/or how to detect them before the efficiency of the electrical machine has been deteriorated or before any part of the machine has been damaged totally.
There have been many approaches to utilize the RF signal detection when defining the appearance of the electric discharges. The RF signal detectors have been proposed for detecting the electric discharges that are caused (e.g., by the loose or broken conductors in the windings, intermittent strand and turn shorts in the windings). Further, there have been several proposals to monitor brush arcing in the commutator of the DC machine.
The electric discharges and the RF signals are in direct connection with the windings and the commutators, respectively, where the discharges can originate. For example, the electric discharge and the RF signals can be generated in commutator because of the wearing and ageing of the brushes.
With the bearing currents, the situation is more difficult. The bearing currents can not be measured in many cases, but other variables, like common mode current or voltage in the terminals of the motor, or the voltage over the bearing may be measured. In some cases even these variables are inaccessible. Further, it is not certain whether harmful discharges occur in the bearings.
Published International Application WO 2007/106015, the disclosure of which is hereby incorporated by reference in its entirety, discloses a method for indicating an electric discharge in a bearing of an electric drive system. The detected RF signals are processed to estimate the electric discharge and the estimated electric discharge is indicated. While known solutions use the RF signals when detecting electric discharges of the electrical machine, they suffer drawbacks with regard to identifying when the detected RF signal truly is connected to the electric discharge in question.